This proposal seeks support to study the structural basis of specific macromolecular interaction typical of the cell surface. The core of the experimental approach will be X-ray diffraction studies of single crystals. The systems to be examined by this approach are (1) Neurophysin-vasopressin complex, a complex between a protein and peptide hormone; (2) delta 5-3-ketosteroid isomerase, a protein that interacts specifically with a steroid hormone; (3) snake venom phosphodiesterase, a protein that recognizes specific phospholipids (crystalline); (4) Non-covalent complex between a lytic fragment of "band 3" protein (human RBC ghosts) and mammalian enzymes; (5) the cholera toxin. In all cases but (4) crystals of these materials have been obtained. BIBLIOGRAPHIC REFERENCE: E.M. Westbrook, P.B. Sigler, H. Berman, J.P. Glusker, G. Bunick, A. Benson and P. Talalay. Characterization of a monoclinic crystal form of an enzyme of steroid metabolism, delta 5-3-ketosteroid isomerase. J. Mol. Biol. 103, 665-667 (1976).